1. Field of the Invention
The present invention relates to DC-DC power converters. In particular, the present invention relates to a self-oscillating DC-DC power converter and circuit, and a method of self-oscillating a DC voltage.
2. Description of the Prior Art
Generally, a control IC is used to control a DC-DC power converter to convert an input DC voltage into a desired output DC voltage (either higher or lower than the input DC voltage). FIG. 1 is a schematic diagram of a circuit for a DC-DC converter controlled by an ASIC UC3842 to convert input DC voltage Vin into output DC voltage Vo. This particular design circuit has some shortcomings. First, when the input voltage is lower than 5V, a DC-DC converter which utilizes an IC for a controller cannot be used because IC""s do not work properly in low voltage input situations. Also, the use of an IC to control the circuit increases the overall cost of the circuit.
To overcome the above-mentioned drawbacks, a self-oscillation DC-DC converter (i.e., a BUCK DC-DC converter) has been proposed by the Chinese Patent No. 99108088.2. This BUCK DC-DC converter is shown in FIG. 2. As shown therein, the DC-DC converter includes a PNP transistor Q1, a primary turn L1, a diode D1 and a capacitor C2. An auxiliary turn L2 connects the base of transistor Q1 to the emitter of transistor Q1 through a capacitor C1 and a resistor R3. The circuit also includes a transistor Q2 which has its emitter and collector connected to the emitter and base of transistor Q1 to conduct a part of the base current for transistor Q1. Transistor Q2 decides whether transistor Q1 is on or off according to the change of output voltage. Therefore, this DC-DC converter keeps the output voltage steady via the use of transistor Q2.
This BUCK DC-DC converter does not adopt the use of an IC controller and is effective for use with a low input DC voltage. Further, since an IC is not used, it costs less to manufacture than that of the ASIC circuit of FIG. 1. However, this BUCK DC-DC converter has a few drawbacks. First, this BUCK DC-DC converter is only suitable with PNP transistors. This DC-DC converter is also only suitable for use where the output voltage is always lower than the input voltage, and cannot be applied for use as a BOOST DC-DC converter wherein the output voltage is higher than the input voltage. Moreover, because this BUCK DC-DC converter requires an auxiliary turn L2 to start oscillation, it requires numerous components and is difficult to manufacture.
Accordingly, there remains a need for a simple DC-DC converter which is self-oscillating and can be used effectively as a BOOST DC-DC converter.
The present invention is a self-oscillation DC-DC converter which utilizes a single turn inductor L1. In the circuit of the present invention, the input positive terminal of the DC source is connected to a terminal of the inductor L1. The other terminal of the inductor L1 is connected to the collector of a transistor Q1 and the anode of a diode D1. One terminal of a capacitor C1 is connected to the output positive terminal Vo+ and the other terminal of capacitor C1 is connected to the output negative terminal Voxe2x88x92. The input negative terminal of the DC source is directly connected to the output negative terminal. The base of transistor Q1 is connected to the input positive terminal of the DC source through a resistor R1. The emitter of the transistor Q1 is connected to the input negative terminal of the DC source. A second transistor Q2 is also provided. The collector of transistor Q2 is connected to the base of transistor Q1 and the emitter of second transistor Q2 is connected to the emitter of transistor Q1. The base of transistor Q2 is connected to the collector of transistor Q1 through a resistor R2.
The self-oscillation DC-DC converter circuit described herein does not use an auxiliary turn, thereby allowing the use of NPN transistors. Positive feedback is achieved by a voltage change Vce1 of the transistor Q1 from a saturation state to a non-saturation state. In the non-saturation state, the transistor Q2 is turned on, the transistor Q1 is turned off and self-oscillation is achieved. The self-oscillation DC-DC converter according to the present invention operates normally when the input voltage is lower than 5Vdc. Also, the use of fewer components lowers the overall cost of the circuit and enables a decreased size circuit to be produced.
The self-oscillation DC-DC converter circuit of the present invention uses relatively few components, thereby decreasing the cost to manufacture and decreasing the overall size required of the circuit. Moreover, the self-oscillation DC-DC converter works when the input voltage is lower than 5Vdc.
A further embodiment is also disclosed wherein power loss is reduced and overall efficiency of the circuit is increased by the addition of a power amplifier driver circuit and a power stage to create a self-oscillation BOOST DC-DC converter.
The power amplifier driver circuit added in the further embodiment of the present invention preferably comprises an NPN transistor Q3, resistors R4, R5 and a capacitor C2. The power stage of the self-oscillation BOOST DC-DC converter includes a high power transistor Q4, an inductor L2, a diode D2 and a capacitor C3. The power amplifier driver circuit amplifies the power of the voltage (pulse signal Vce1) from the collector to the emitter of transistor Q1 to drive the high power transistor Q4. Resistor R4 and capacitor C2 trim the pulse signal Vce1. Transistor Q4 boosts the input voltage to a higher output voltage. When transistor Q4 is turned on, inductor L2 is charged. When transistor Q4 is turned off, the charge stored in inductor L2 is discharged to output. The high power transistor Q4 is preferably a power MOSFET and reduces power dissipation.
With the addition of the power amplifier driver circuit and the high power transistor, the self-oscillation DC-DC converter of the present invention is adapted to be a self-oscillation BOOST DC-DC converter. Accordingly, a highly efficient self-oscillation BOOST DC-DC converter is achieved.